1. Field of the Invention
This invention relates to the production and purification of metals. In particular, this invention relates to production (extraction) from ore, refining from metal alloy, and recycling from metal matrix composites, of lightweight and reactive metallic elements using electrolysis in electrolytes at or near room temperature.
2. Description of the Related Art
Conventional production, refining and recycling of lightweight and reactive metallic elements involves high temperature electrolysis in molten salts. Aluminum provides an illustrative example.
For the past century, primary aluminum has been produced using the Bayer-Hall-Heroult process, which involves high temperature electrolysis of alumina dissolved in molten cryolite (Na3AlF6). Current aluminum refining also uses high temperature electrolysis.
However, conventional high temperature electrolysis processes require the use of many expensive refractory materials. In addition, conventional high temperature processes consume large amounts of energy. Current high temperature refining of primary and recycled aluminum uses three-layer electrolysis with even higher energy consumption than primary aluminum production. High temperature electrolysis processes also produce large amounts of pollutants. CF4 gas formed in current industrial processes has an extremely long atmospheric residence time and a very high “global warming potential” (about 5100 times higher than CO2). Thus, high temperature electrolysis processes have several disadvantages, including the use of expensive refractory and electrode materials, high production costs and high pollutant emission.
To overcome these disadvantages, electrodeposition processes at or near room temperature have been explored.
U.S. Pat. Nos. 2,446,331; 2,446,349 and 2,446,350 disclose room temperature electrodeposition of aluminum from a molten electrolyte consisting of an aluminum salt, such as aluminum chloride, and an ionic liquid of a N-alkyl pyridinium halide, such as ethyl pyridinium chloride. However, the ethyl pyridinium chloride has the disadvantage of dissolving deposited aluminum and decreasing current efficiency.
U.S. Pat. Nos. 4,624,753; 4,624,754; and 4,624,755 also disclose room temperature electrodeposition of metals, using as ionic liquids non-aqueous nitrate-amide melts to electrodeposit metals such as Fe, Ni, Zn, Ag, Pb and Cu. However, the “electrochemical window” (i.e., difference between the lower and upper voltage limits for electrodeposition) of the nitrate-amide ionic liquids is not high enough for the electrodeposition of aluminum.
U.S. Pat. No. 5,552,241 discloses low temperature molten salt compositions comprised of a mixture of a metal halide, such as aluminum trichloride, and fluoropyrazolium salt.
U.S. Pat. No. 5,731,101 discloses a low temperature molten composition comprising a mixture of a metal halide and an alkyl-containing amine hydrohalide salt.
U.S. Pat. No. 5,855,809 discloses electrolytes, which do not crystallize at ambient temperature, formed by the reaction of a strong Lewis acid, such as AlCl3, with an inorganic halide-donating molecule.
Although room temperature production and refining processes have met with some success, the recycling of aluminum from composites of refractory particles in a matrix of aluminum alloy is much more difficult technologically. Aluminum alloy composites are finding increasing application. For example, discontinuously reinforced aluminum (“DRA”) composites are finding extensive use in the automotive industry. However, when DRA is re-melted along with regular aluminum alloy, the recycled product is very difficult to be fabricate because of the very high hardness resulting from the presence of refractory reinforced particles, such as silicon carbide. To separate the refractory particles from the aluminum alloy, it is common to filter the particles from a melt of the recycled product. However, high temperature filtration processes are very difficult to perform.
Because of the difficulties encountered in conventional processes, there is a need for improved methods of producing, refining and recycling lightweight and reactive metallic elements.